A great effort is underway to integrate optical and electronic technology for optical fiber communication. An optical transmission system is generally made up of an optical source (e.g. a light-emitting diode or laser), an optical fiber and a detector. Small, efficient and reliable, the semiconductor laser has been accepted as a practical light source for the optical fiber communication.
Vertical Cavity Surface Emitting Laser diode, hereinafter referred to as a VCSEL, is attractive as a device which may be produced by planar technology and as a class of devices with a wide range of potential uses including optical communications, optical discs, laser printers and light sensing systems. In the VCSEL the lasing cavity is perpendicular to the top surface of a laser chip. Therefore, high packing density, compared to the packing density of edge-emitting lasers with lasing cavity parallel to the surface of the laser chip, is obtainable. This would lead to a promising future in high density laser arrays, high data transmission in optical communication systems, ultra high parallel processing in optical communication systems, as well as supplying a route for fast and vast data transmission between electronic chips. Furthermore, the circular-like nature of their beams makes them suited for beam-combining for high-power applications.
In the VCSEL the light output is in the film growth direction which is usually parallel to the direction of the injection current. Due to this feature, the mirror and electrical contact physically occupy the same side of the laser structure, i.e. either on the top (front) or on the bottom (rear) of the device. Typically, the mirror is located approximately in the center of the top surface while the electrode is located peripherally of the mirror. In some designs, the front mirror and the electrode are coplanar while in some others the planes are vertically offset each from another. An example of a coplanar arrangement may be found in articles by H. Soda et al., entitled "GaInAsP/InP Surface Emitting Injection Lasers," Japanese Journal of Applied Physics, Vol. 18, No. 12, 1979, pp. 2329-2230; and by H. Soda et al. entitled "GaInAsP/InP Surface Emitting Injection Lasers with Short Cavity Length," IEEE Journal of Quantum Electronics, Vol. QE-19, No. 6, June 1983, pp. 1035-1041. Each of these articles describes a surface emitting laser with a coplanar mirror/electrode arrangement and a Au mirror a few tenths of micrometer thick. However, S. Kinoshita pointed out that such mirrors lead to low quantum efficiency. See an article by Susumu Kinoshita et al. entitled "GaAlAs/GaAs Surface Emitting Laser with High Reflective TiO.sub.2 /SiO.sub.2 Multilayer Bragg Reflector," Japanese Journal of Applied Physics, Vol. 26, No. 3, March 1987, pp. 410-415. This led to the development of laser structures utilizing multilayer (semiconductor or dielectric) mirrors. For example, see L. M. Zinkiewicz et al., "High Power Vertical-Cavity Surface-Emitting AlGaAs/GaAs Diode Lasers," Appl. Phys. Letters, Vol. 54, No. 20, May 15, 1989, pp. 1959-1961, and Kenichi Iga, "Recent Advances of Surface Emitting Semiconductor Lasers," Optoelectronics-Devices and Technologies, Vol. 3, No. 2, December 1988, pp. 131-142.
However, these structure are complicated and cumbersome. An attempt to simplify construction of a VCSEL by combining the mirror and the electrode into a single unit led to relatively low quantum efficiencies. The mirrors comprised an 0.55 .mu.m thick reflective Ag mirror which also operated as the electrode of the laser. The emission took place through the .lambda./4 reflector stack consisting of 23 pairs of semiconductor layers, the semiconductors having different refractive indices within each pair. See Deppe D. G., et al., "AlGaAs-GaAs and AlGaAs-GaAs-InGaAs vertical cavity surface emitting lasers with Ag mirrors," Journal of Applied Physics, Vol. 66, No. 11, December 1989, pp. 5629-5631.
Therefore, there is still a need for a VCSEL with an improved quantum efficiency which could be also produced in a simplified manner utilizing planar technology.